One of the reasons that the US has maintained a presence in Japan, Okinawa and South Korea for the past now nearly seventy years, is as a listening and intelligence-gathering outpost. There are few, if any, actual spies on the ground, but units of the various deployed military elements of North Korea and China communicate with each other via radio, and there are intercept operators on duty listening in to the chatter between these units. This activity is not exactly secret, as both sides engage in such clandestine listening and interpretation, but the actual intercept methods and the information derived are classified very highly. That is, of course, unless some member of Congress on any of the various Intelligence committees or subcommittees so chooses to disclose these sources and the raw intelligence derived from this endeavor, as has happened a number of times in the past.
I know, this is very close to “Loose Lips Sink Ships”, but nothing I have said here reveals anything not already known widely among the rival nations who also maintain such presence. “The information is classified so highly, only the enemy will ever find out”, is a statement often repeated within the intelligence community.
So when somebody asks why, a half-century or more after the end of WW II, American soldiers are still deployed at forward areas of potential conflict, that is the reason. Calls for bringing them home are actually providing aid and comfort to real and potential adversaries.
A late friend of mine was a “listener” in the US Army in Berlin back in the day. He spoke fluent German and could understand the slang.